The present invention relates to a control system for an automatic transmission, which is suitable, particularly, for start control of a V-belt type continuously variable transmission (CVT) at an ultra-low temperature.
In motor vehicles including an automatic transmission, a hydraulic system for transmission forms a drive load on a starter motor at engine start. At an ultra-low temperature of −20to −40° C., particularly, the oil viscosity is higher to increase the resistance of the whole hydraulic system, decreasing cranking rotational speed, leading to degradation of the engine start-ability.
In order to prevent such inconvenience, a typical control system for automatic transmission sets provisionally an indicated value of the line pressure at a minimum when a detected oil temperature is low at engine start, reducing a drive load on an oil pump, thus enhancing the engine start-ability. And when selecting a drive range during the minimum line-pressure control, the indicated value of the line pressure is set at a maximum as soon as the engine speed reaches a predetermined value, increasing the line pressure in a stroke, supplying the hydraulic pressure to engagement elements.
The maximum value of the line pressure is created by draining or reducing the source pressure of the oil pump to a “design maximum value” through a pressure regulating valve. The typical control system, which aims to enhance only the engine start-ability at the time of ultra-low temperature start, sets an indicated value of the line pressure at a maximum as soon as the engine speed reaches a predetermined value, having possible occurrence that the indicated value reaches a maximum before regulating the line pressure.
In that case, since the pressure regulating valve has a drain port closed before regulating the line pressure, and is not supplied with the hydraulic pressure enough to reduce the source pressure to the “design maximum value”, the engagement elements of the transmission undergo a greater pressure than the “design maximum value”. That is, an overshoot of the line pressure becomes greater with respect to the “design maximum value”.
Typically, the strength of the engagement elements of the transmission, particularly, pulleys of the V-belt type transmission, is designed to have the “design maximum value” as an upper limit. Thus, when undergoing a greater pressure than the “design maximum value”, the pulleys suffer deterioration of the durability.